An expert physician has extensive knowledge about the physiological mechanisms of the body. The cognitive structure of this knowledge and its role in clinical problem-solving are poorly understood. A better understanding of the actual structure of medical knowledge as applied by expert physicians could lead to improved methods of medical education, and to new computer tools to improve the quality and lower the cost of health care delivery. We propose to conduct an empirical and theoretical investigation of the expert physician's knowledge of physiological mechanism. We hypothesize that the physician's knowledge of physiological mechanism functions as a cognitive "causal model" of the patient, which can be used to simulate the behavior of the patient under different assumptions. We will apply techniques of protocol analysis derived from information-processing psychology, and techniques of computational modelling derived from artificial intelligence to propose representations of knowledge capable of accounting for the observed behavior.